The present invention relates to thermal printers and more particularly to a drive circuit for amplifying logic level signals into high current pulses.
In prior art thermal print drivers, a DTL (Diode Transistor Logic) gate is used in a circuit to drive a complementary bipolar transistor pair which provides current to a resistive heating element. This circuit suffers from disadvantages in that it consumes an excessive amount of power and looses significant power handling capability at lower temperatures. It also has a relatively low overall power output which causes the system to print at a comparatively slow rate of speed due to the time required to provide sufficient power to heat the thermal printing element. Therefore, there is a need for a printer driver circuit which overcomes these problems.
In a thermal printer, an electrical current is supplied to a resistive heating element which is in contact with thermal sensitive paper. When the element is heated by the current, it forms a dot on the paper. For high speed operation of the printer, the heating element must be supplied with a high current pulse to rapidly heat the element and cause a dot to be printed on the paper. The resistive heating element must have a relatively low thermal time constant so that it cools quickly and does not produce a smeared impression on the paper. Therefore, for optimum operation, the heating element must be supplied with a high, but short time period, current pulse.
A thermal printer heating element prints a series of dots in order to form characters on the thermal sensitive paper. The commands to print the dots are generated by digital logic circuitry, primarily CMOS logic which is widely used due to its low cost and low power consumption. However, the output of a CMOS logic is a relatively low voltage, high impedance signal which must be substantially amplified to provide sufficient current to the resistive heating element. In addition, the drive circuit must be relatively insensitive to temperature changes in order to meet military temperature specification requirements.
In accordance with the present invention there is provided a thermal print driver circuit which provides a heavy current driver pulse, as much as several amps, in response to a CMOS logic level signal, has a rapid rise time, and continues to supply high current pulses even at extremely low temperatures.